The Story of the Heavens - Sir Robert Stawell Ball (ebook reader for laptop .TXT) 📗

this case the sunlight illuminates one side of the ring, while it is the other side of the ring that is presented towards the earth. Powerful telescopes are necessary to deal with the planet under such circumstances; but it may be reasonably hoped that the questions relating to the division of the ring, as well as to many other matters, will then receive some further elucidation.

Occasionally, other divisions of the ring, both inner and outer, have been recorded. It may, at all events, be stated that no such divisions can be regarded as permanent features. Yet their existence has been so frequently enunciated by skilful observers that it is impossible to doubt that they have been sometimes seen.

It was about 200 years after Huyghens had first explained the true theory of Saturn that another very important discovery was effected. It had, up to the year 1850, been always supposed that the two rings, divided by the well-known black line, comprised the entire ring system surrounding the planet. In the year just mentioned, Professor Bond, the distinguished astronomer of Cambridge, Mass., startled the astronomical world by the announcement of his discovery of a third ring surrounding Saturn. As so often happens in such cases, the same object was discovered independently by another--an English astronomer named Dawes. This third ring lies just inside the inner of the two well-known rings, and extends to about half the distance towards the body of the planet. It seems to be of a totally different character from the two other rings in so far as they present a comparatively substantial appearance. We shall, indeed, presently show that they are not solid--not even liquid bodies--but still, when compared with the third ring, the others were of a substantial character. They can receive and exhibit the deeply-marked shadow of Saturn, and they can throw a deep and black shadow upon Saturn themselves; but the third ring is of a much less compact texture. It has not the brilliancy of the others, it is rather of a dusky, semi-transparent appearance, and the expression "crape ring," by which it is often designated, is by no means inappropriate. It is the faintness of this crape ring which led to its having been so frequently overlooked by the earlier observers of Saturn.

It has often been noticed that when an astronomical discovery has been made with a good telescope, it afterwards becomes possible for the same object to be observed with instruments of much inferior power. No doubt, when the observer knows what to look for, he will often be able to see what would not otherwise have attracted his attention. It may be regarded as an illustration of this principle, that the crape ring of Saturn has become an object familiar to those who are accustomed to work with good telescopes; but it may, nevertheless, be doubted whether the ease and distinctness with which the crape ring is now seen can be entirely accounted for by this supposition. Indeed, it seems possible that the crape ring has, from some cause or other, gradually become more and more visible. The supposed increased brightness of the crape ring is one of those arguments now made use of to prove that in all probability the rings of Saturn are at this moment undergoing gradual transformation; but observations of Hadley show that the crape ring was seen by him in 1720, and it was previously seen by Campani and Picard, as a faint belt crossing the planet. The partial transparency of the crape ring was beautifully illustrated in an observation by Professor Barnard of the eclipse of Iapetus on November 1st, 1889. The satellite was faintly visible in the shadow of the crape ring, while wholly invisible in the shadow of the better known rings.

The various features of the rings are well shown in the drawing of Trouvelot already referred to. We here see the inner and the outer ring, and the line of division between them. We see in the outer ring the faint traces of the line by which it is divided, and inside the inner ring we have a view of the curious and semi-transparent crape ring. The black shadow of the planet is cast upon the ring, thus proving that the ring, no less than the body of the planet, shines only in virtue of the sunlight which falls upon it. This shadow presents some anomalous features, but its curious irregularity may be, to some extent, an optical illusion.

There can be no doubt that any attempt to depict the rings of Saturn only represents the salient features of that marvellous system. We are situated at such a great distance that all objects not of colossal dimensions are invisible. We have, indeed, only an outline, which makes us wish to be able to fill in the details. We long, for instance, to see the actual texture of the rings, and to learn of what materials they are made; we wish to comprehend the strange and filmy crape ring, so unlike any other object known to us in the heavens. There is no doubt that much may even yet be learned under all the disadvantageous conditions of our position; there is still room for the labour of whole generations of astronomers provided with splendid instruments. We want accurate drawings of Saturn under every conceivable aspect in which it may be presented. We want incessantly repeated measurements, of the most fastidious accuracy. These measures are to tell us the sizes and the shapes of the rings; they are to measure with fidelity the position of the dark lines and the boundaries of the rings. These measures are to be protracted for generations and for centuries; then and then only can terrestrial astronomers learn whether this elaborate system has really the attributes of permanence, or whether it may be undergoing changes.

We have been accustomed to find that the law of universal gravitation pervades every part of our system, and to look to gravitation for the explanation of many phenomena otherwise inexplicable. We have good reasons for knowing that in this marvellous Saturnian system the law of gravitation is paramount. There are satellites revolving around Saturn as well as a ring; these satellites move, as other satellites do, in conformity with the laws of Kepler; and, therefore, any theory as to the nature of Saturn's ring must be formed subject to the condition that it shall be attracted by the gigantic planet situated in its interior.

To a hasty glance nothing might seem easier than to reconcile the phenomena of the ring with the attraction of the planet. We might suppose that the ring stands at rest symmetrically around the planet. At its centre the planet pulls in the ring equally on all sides, so that there is no tendency in it to move in one way rather than another; and, therefore, it will stay at rest. This will not do. A ring composed of materials almost infinitely rigid might possibly, under such circumstances, be for a moment at rest; but it could not remain permanently at rest any more than can a needle balanced vertically on its point. In each case the equilibrium is unstable. If the slightest cause of disturbance arise, the equilibrium is destroyed, and the ring would inevitably fall in upon the planet. Such causes of derangement are incessantly present, so that unstable equilibrium cannot be an appropriate explanation of the phenomena.

Even if this difficulty could be removed, there is still another, which would be quite insuperable if the ring be composed of any materials with which we are acquainted. Let us ponder for a moment on the matter, as it will lead up naturally to that explanation of the rings of Saturn which is now most generally accepted.

Imagine that you stood on the planet Saturn, near his equator; over your head stretches the ring, which sinks down to the horizon in the east and in the west. The half-ring above your horizon would then resemble a mighty arch, with a span of about a hundred thousand miles. Every particle of this arch is drawn towards Saturn by gravitation, and if the arch continue to exist, it must do so in obedience to the ordinary mechanical laws which regulate the railway arches with which we are familiar.

The continuance of these arches depends upon the resistance of the stones forming them to a crushing force. Each stone of an arch is subjected to a vast pressure, but stone is a material capable of resisting such pressure, and the arch remains. The wider the span of the arch the greater is the pressure to which each stone is exposed. At length a span is reached which corresponds to a pressure as great as the stones can safely bear, and accordingly we thus find the limiting span over which a single arch of masonry can be constructed. Apply these principles to the stupendous arch formed by the ring of Saturn. It can be shown that the pressure on the materials of the arch capable of spanning an abyss of such awful magnitude would be something so enormous that no materials we know of would be capable of bearing it. Were the ring formed of the toughest steel that was ever made, the pressure would be so great that the metal would be squeezed like a liquid, and the mighty structure would collapse and fall down on the surface of the planet. It is not credible that any materials could exist capable of sustaining a stress so stupendous. The law of gravitation accordingly bids us search for a method by which the intensity of this stress can be mitigated.

One method is at hand, and is obviously suggested by analogous phenomena everywhere in our system. We have spoken of the ring as if it were at rest; let us now suppose it to be animated by a motion of rotation in its plane around Saturn as a centre. Instantly we have a force developed antagonistic to the gravitation of Saturn. This force is the so-called centrifugal force. If we imagine the ring to rotate, the centrifugal force at all points acts in an opposite direction to the attractive force, and hence the enormous stress on the ring can be abated and one difficulty can be overcome.

We can thus attribute to each ring a rotation which will partly relieve it from the stress the arch would otherwise have to sustain. But we cannot admit that the difficulty has been fully removed. Suppose that the outer ring revolve at such a rate as shall be appropriate to neutralise the gravitation on its outer edge, the centrifugal force will be less at the interior of the ring, while the gravitation will be greater; and hence vast stresses will be set up in the interior parts of the outer ring. Suppose the ring to rotate at such a rate as would be adequate to neutralise the gravitation at its inner margin; then the centrifugal force at the outer parts will largely exceed the gravitation, and there will be a tendency to disruption of the ring outwards.

To obviate this tendency we may assume the outer parts of each ring to rotate more slowly than the inner parts. This naturally requires that the parts of the ring shall be mobile relatively to one another, and thus we are conducted to the suggestion that perhaps the rings are really composed of matter in a fluid state. The suggestion is, at first sight, a plausible one; each part of each ring would then move with an appropriate velocity, and the rings would thus exhibit a number of concentric circular currents with different velocities. The mathematician can push this inquiry a little farther, and he can study how this fluid would behave under such circumstances. His symbols can pursue the subject into